The bug

The blur shader is still used in Programmer Art and High Contrast, as well as other resource packs using opaque UI backgrounds. We can tell this is the case due to MC-268902.

How to reproduce

1. Open the debug menu with performance graphs via F3+2
2. Enable the Programmer Art resource pack
3. Observe how the graph at the left looks after unpausing the game

Expected results

The reported frames per second would not be lower when the game is paused.

Actual results

They are, implying the blur shader is being used even though it isn't visible.

How to fix

If the menu background nor the menu list background are found to have an alpha channel, it might be advisable to cancel use of the blur shader in a menu context entirely.

Alternatively, include a shader in Programmer Art that does nothing, and have it replace the blur shader.